1. Field of the Invention
The present invention relates generally to image processing for electronic imaging sensors, and more particularly, to an apparatus and method for equalizing the dark current in pixels located on the periphery of an image sensor and thereby eliminating the so-called “edge effect” to optimize image quality.
2. Description of the Related Art
Optical black pixels (“OB pixels”) are the reference pixels within an imaging sensor pixel array that should have virtually identical electrical characteristics as a standard active pixel, yet are insensitive to light. OB pixels are used by various image processing circuits in the sensor or camera to maintain a constant black level independent of gain, illumination, changes in sensor operating conditions, etc. An OB pixel's signal is normally processed using the same signal chain and timing as those that are photo-electrically responsive and form the image of interest. The latter pixels, which constitute the majority of pixels in an imaging sensor, are some times referred to as “Clear Pixels.” While clear pixels form the actual image, OB pixels are indirectly used in the image formation process. Instead, OB pixels are used to track the environmental and operational status of the imaging sensor while also providing information regarding dark signal/current, column noise and line noise at the specific location of each OB pixel.
In an imaging System-on-Chip (iSoC) system, OB pixels are typically used as reference pixels for many calibration operations that enhance image quality. These include black clamp stabilization, flutter rejection, column noise suppression and line noise correction. In these and other algorithms, OB pixels must have similar dark signal response as a clear pixel; they must mimic the response that a clear pixel would normally have in the absence light to accurately establish the current vs. voltage (i-v) characteristic for general reference in the image sensor.
There is also a need for both clear and OB (i.e., opaque) pixels that are not necessarily read out to support signal processing operations. These so-called “dummy pixels” nevertheless help increase image quality by reducing edge effects that are either deleterious to the image processing or perturb clear and OB pixels.
In general, the i-v characteristic for a diode is described by the Shockley equation:I=Io(eqvd/nkt−1)I=Io(eqvd/nkt−1)Where Io is the diode's saturation current, Vd is the diode voltage, n is the diode ideality, k is Boltzman's constant and T is the Temperature. Depending on the local environment, processing conditions, etc., the saturation current can vary significantly from diode to diode. To account for practical issues, the Shockley equation can be alternately expressed:
      I    =                  n        d            ⁢              kT        /        q            ×                        A          d                                      R            o                    ⁢          A                    ×              (                              ⅇ                                                            qV                  d                                /                                  n                  d                                            ⁢              kT                                -          1                )                  I    =                  n        d            ⁢              kT        /        q            ×                        A          d                                      R            o                    ⁢          A                    ×              (                              ⅇ                                                            qV                  d                                /                                  n                  d                                            ⁢              kT                                -          1                )            Where nd is the diode ideality for the specific diode and the parameter RoA is its resistance-area product at zero volts of applied bias, expressed in units of ohm-cm2. While diode resistance and saturation current depend on area, the RoA product for a specific technology is independent of area and characterizes the quality of diode fabrication.
Different applications of an imaging sensor pixel array may require differing degrees of image quality. For instance, a CMOS sensor used in a studio broadcast camera to generate content for HDTV production (high-end application) requires significantly higher image quality than a CMOS sensor used in consumer camcorders (medium to low-end product application). In addition to the well known attributes such as sensitivity and low noise, other quality factors include drift-free operation and insensitivity to excess illumination. Maximizing image quality hence entails not only better clear pixel quality, but also better OB quality. Consequently, a critical property of an OB pixel (active or dummy) is how closely it truly represents the dark signal response of a clear pixel regardless of incident lighting and environment including physical and electrical stress. For instance, high end applications require an OB pixel to have significantly higher fidelity under extreme conditions, e.g., superior opacity, thermal stability, and well-behaved electrical characteristics including detector ideality, nd, and RoA product.
In addition to the difficulty in achieving the necessary opacity in OB pixels without perturbing electrical characteristics, there is also a need for forming dummy pixels that generate the same nominal current as OB pixels and clear pixels. The total current in clear, dummy and an OB pixel is the sum of each diode's Shockley-Read current, photocurrent and the electrical bias current; it is expressed as the direct sum:
      I    total    =      I    -          I      photo        +                  V        d                    R        d            where Rd is the detector's resistance at the applied bias, Vd. The total current accumulated in clear pixels versus that in OB pixels should differ only by the clear pixel photocurrent, Iphoto; Iphoto must be zero in OB and OB dummy pixels.